Method of making plungers for glass molds



Dec. 17, 1935. R. w. LUCE METHOD OF' MAKINGk PLUNGERS FOR GLASS MOLDS Filed Feb. 2l, 1953 Patented Dec. 17, 1935 'UNITED STATES METHOD oF MAKiNG PLUNGERS Foa GLASS Moms Richard W. Luce, Elizabeth, N. J., assigner to` Eastman Kodak Company,` Rochester, N. Y., a corporation of New York Application February 21, 1933, Serial No. 657,832,

1 Claim.

This invention relates to the molding of glass lenses and particularly to an improved method of making hobs or plungers for molds to be used for the molding of small lenses such as those used in reflecting sign When the refracting surface of a lens is molded in its finished shape i. e., it is not thereafter polished, the surface of the mold must be given a very high polish. When it is desired to mold a lens having a spherical surface the high polish necesy sary to obtain a satisfactory optical surface may be given the mold byordinary' pitch polishing methods and this pitch polishing may be applied directly to the surface of the mold or to a hob which is to be used to sink th`e mold. Heretofore, a small lens having a refracting surface other than a single spherical surface could not be molded satisfactorily because'of the diiliculty, if not impossibility, of accurately polishing the mold or the hob used to form the mold since pitch polishing methods could not be used.

The object of this invention is to provide a method of making hobs with whichrsurfaces of shapes other than a single sphere may be ob. tained which have a high polish whereby glass lenses having truly optical surfaces can be completely finished directly in the mold. In accordance with this object the hob used to form the mold is made and polished in sections whereby ordinary pitch polishing methods may .be employed.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claim. My invention itself, however, will best be understood by reference to the following description taken in connection with the accompanying drawing in which Figure 1 is a diagram showing one type of lens which may be formed with a mold made in accordance with-the method of my invention;

Figure 2 is a section of a -mold from which the` lens of Figure 1 may be formed;

Figure 3 is an elevation of a rod forming the central part of a hob;

Figure 4 is a section of a tube forming an annular part. of the hob and is shown containing a core which may be used to facilitate accurate polishing; l

Figure 5 is a sectional view of a complete hob assembled from parts including those illustrated in Figures 3 and 4; 4 4

Figure 6 is a sectional view illustrating a suitable arrangement which may be used in sinking the mold with the hob;

Figure 'I shows another lens surface which may be molded; and l Figures 8 and 9 show how a hob corresponding to the lens surface of Figure .7 may be made in accordance with this invention. e

(Cl. 'ze-107) 1 Referring to the drawing, Figure 'i shows a lens II) consisting of a single piece ofglass have ing a plane rear surface Il and a, frusto conical side l2. 'The from; of the lens It consists of a series of co-axial contiguous spherical zones I5.`

the curvatures of which are so Vcomputed that the average focus for the central part of each zone lies at a common point, all of which is fully described and claimed in co-pending application Serial Number 639,575 led October 26,A 10 Y 1932, by Charles W. Frederick and assigned to the same assignee as is the present invention.

In forming such-a lenswitlr a mold such-as is shown in Figure'Z it is obvious that the inner face I3 of the mold formed in a cylindrical block 15 I4 of nickel or nickel alloy must have a high polish if the lens I0 is to have a true optical surface, since the different curvatures of the front of the 1ens lo cannot later bepolishea by orda. nary pitch polishing methods.

A hob used to 20 form the mold I4 having a series of surfaces of different curvatures if comprised of a single block of metal, is equally impossible to polish with any degree of accuracy and accordingly some method must be found either for accu- 25 rately polishing the molding surface I3 orfor polishing a hob used to sink the mold. My invention furnishes a practical and a simple method whereby pitch polishing may be successfully utilized in providing a molding surface 30 having aV high polish.

My invention is equally applicable to polishing the mold directly or polishing the hob used to sink the mold and will be described in detail as applied to the polishing of the hob. 35

As shown in Figure 3 a central rod I5 of steel or other suitable metal has a convex endIS which is polished Aby regular optical polishing methods onja pitch polisher to the radius of curvature desired for the central zone of the 40 lens I0 to be molded, the diameter of the rod I5 being the diameter of the central zone.1 The part of the hob corresponding to the next outer zone of the lens is now made from` a tube I1,

a cross section of which is shown in Figure. 4, 45 having an internal diameter' equal to the diameter of the rod I5 and having an external diameter equal to the external diameter of the 2nd zone of the lens I0. One end of the tube I1 is then subjected to a grinding and polish- 50 ingI operation to form a spherical zone I8 corresponding to the curvature desired for this zone in the finished lens. This isv also done by a regular pitch polishing method and the surface I8 is given a true optical polish. Inasmuch 55 as in the forming of a mold for making relatively small lenses s'uch, for example, as those' now generally used in highway reflecting signs the lwall of the tube I1 is not very thick and it is. generally desirable, but not necessary, to insert a core I9 which may be of any suitable metal, or wood, in the tube I1 for the polishing operation. The core I9 provides a more substantial surface for polishing and renders the centering of the polishing apparatus much easier than when it is attempted to polish the surface I3 without the core I9 being present. The same operation is then repeatedy for the number of zones desired and the several tubes are then assembled with the rod I5 to form a hob 26 as shown in section in Figure` 5 where there is shown the central rod I5 and a series of tubes I1, 20 and'2I having spherical surfaces I6, I8, 22, and 23 leach of whichk has been finished by regular optical methods and accordingly each has the degree of polish obtained by such methods.

If it is desired, as is 'quite usual, the outermost tube 2| may be provided with a sloping outer surface 24 to facilitate removal of a lens from the mold and with a thickened base 25 to make it more sturdy. It will b'e understood that the length of the rod I5 and the tubes I1, 20, and 2I are made such that their spherical surfaces will form a series of contiguous zones unless it is desired to provide offsets between the Zones in which case their individual lengths will bemade accordingly and the resulting lens Will 'be of the type shown in Fig. '7.

With the hob 26 constructed as describedj some means is necessary to insure` that in sinking the mold a proper alinement will be maintained between. the mold blank and the hob, and one suitable arrangement is illustrated in Figure 6 as comprising a heavy base plate 21 provided with a cylindrical recess 28 for receiving and holding the blank mold block I4. The

base plate 21 may be provided with anopening 29 through which .a suitable tool may be inserted to remove the block I4 from the recess 28. The plate 21 is also `provided with at least two guide pinsr30 which are adapted to coopcrate with cylindrical openings 3I in an annular ring 32 to position accurately the ring 32 with respect to the plate 21 carrying the mold blank I4 The hob 26 is given rigidity by mounting it by means of a pressed nt in an annular ring 33 to which is secured a backing plate 34 by any suitable means, as screws 35. The annular' ring 33 carrying the hob 26 is mounted'by means of a pressed t in the annular ring 32 provided with the positioning means 3|. These pressedfittings are satisfactory since the pressure needed to sink the mold is applied, as indicated by the arrow, against the backing plate 34.

When it is desired to sink the mold the blank I 4, which preferably has been provided with a concavity 36 approximating the shape of the mold to be formed, is positioned in the recess 28 and the ring 32 is lowered over the guide pins 30 until the hob 26 rests against the bottom of the concavity 36 in the blank I4. Pressure suflicient to cause the mold to take the shape of the surface of the hob 26 is now applied to the plate 34. I have found that with the blank I4 made of nickel a pressure of 20 to 25 tons is sufficient to make the mold, although other pressuresmay be used and the pressure which will give the best results for different metals may be determined by experiments.

My invention will now be described as applied to the making of a mold for forming lenses having zones other than spherical. One such lens is shown in Figure 7 as comprising a lens 40 having a plane polished rear surface 4I and a convexf front surface consisting of a central spherical zone 42 and two annular torio zones 43 and 44. The lens 40 as shown is provided 5 with offsets between the several zones but they may be contiguous as were the zones of the lens I0 shown in Figure 1.

As in the case previously described, a central rod 45 is provided with a polished spherical sur- 10 face 46 corresponding to the central spherical zone of the lens to be molded. A tube 41 of the proper diameter is then selected and subjected to a grinding and polishing operationso as to form upon it a toric zone 48 corresponding to the 15 desired zone 43 of the finished lens. This can be done by a well known pitch polishing method and the surface is given a true optical polish. Broken line 49 indicates the outline of a core if one is used in the polishing operation. The same 20 operation is repeated for the number of zones desired and the several tubes are assembled with the rod 45 to forma hob which is then used to sink a mold as above described in detail. A lens having such steps or offsets and having zones 25 some of which are not spherical is disclosed in my\eopending application Serial No. 657,831 led February 2l,1933.

In practicing the method of this invention it has been found that when a hob having a high 30 polish is used to sink the mold the active surface of the mold has a polish comparable to the polish on the hob and lenses molded therein have surfaces which are` truly optical.

In making a mold such-as is shown in Figure 35 L. 2 having a series of spherical zones t ispossible to polish the surfaces directly if sulcient care is exercised. The successive surfaces aref'lrst approximated and then each surface is subjected to an individual polishing operation' by means 40 of a tool having a curvature such that it contacts only the zone surface having the same curvature. This individual polishing is possible because the central zone has the smallest radius of curva.- "s ture and each successive zone from th center/45 has an increasingly larger radius. i

Although I have described my invention asv applied to the making of two special forms of molds it is obviously useful in making molds for a great number of other lens forms in whichsiml- 50 lar problems arise. Various modifications will readily suggest themselves to persons skilled in the art without departing from the scope of my invention as set forth in the appended claim.

What I claim as new and desire to secure by 55 Letters Patent of the United States is:

'Ihe method of making plungers for glass molds for producing lenses having concentric zones of different curvatures which comprises forming on one end of a circular rod a. polished surface cor- 60 responding in size and curvature to the central zone of the lens to be formed, providing with a core a tube having an internal diameter substantially equal to the diameter of the rod, ma-

` chining and polishing one end of the cored tube lens to be formed.

RICHARD W. LUCE. 

